Schmitt trigger circuit with low input current

ABSTRACT

A Schmitt trigger circuit has an input-voltage hysteresis characteristic for reducing noise sensitivity and preventing oscillation. In its input stage there is a multi-emitter transistor, and in its output stage a second transistor. The multi-emitter transistor comprises a first emitter and a second emitter. The first emitter is associated with a switching operation in response to the input voltage applied to the base of the multi-emitter transistor. The second emitter is associated with the operation of drawing charges from the base of the second transistor through the base of the multi-emitter transistor to the ground. The use of the multi-emitter transistor prevents the input current from increasing greatly as the input voltage falls.

BACKGROUND OF THE INVENTION

This invention relates to a Schmitt trigger circuit in which the input threshold voltages associated with the rise or fall of the output voltage have a hysteresis characteristic, that is, the low input threshold voltage (V_(L)), when the input voltage varies from a high level (H) to a low level (L), is different from the high input threshold voltage (V_(H)) when the input voltage varies from the L to the H, and more particularly, to a Schmitt trigger circuit in which the input current does not increase suddenly when the input voltage falls from H to L.

A Schmitt trigger circuit, having a high input threshold voltage (V_(H)) associated with the rise of the input voltage and a low input threshold voltage (V_(L)) associated with the fall of the input voltage, has the advantages of reduced noise sensitivity and of no oscillation near the point of the threshold voltages due to the hysteresis characteristic of the input threshold voltages, as hereinafter described in detail with respect to conventional Schmitt trigger circuits. Therefore, such a Schmitt trigger circuit is advantageously used as an input buffer for various logic circuits, such as flip-flops, shift registers, counters, etc., because the input of the input buffer usually tends to receive noise and is connected to a long bus line along which the input waveform tends to deteriorate.

However, in a conventional circuit, there is the disadvantage of a sudden increase in the input current when the input voltage falls from the high input threshold voltage (V_(H)) to the low input threshold voltage (V_(L)). Because of this sudden increase in the input current, a drive circuit for drawing the input current from the input stage of the Schmitt trigger circuit must have the capability of drawing a larger current than the usual input current.

SUMMARY OF THE INVENTION

Accordingly, an object of the present invention is to provide a Schmitt trigger circuit having a hysteresis characteristic in which the input current does not greatly increase.

Another object of the present invention is to provide a Schmitt trigger circuit in which a multi-emitter transistor is employed in the input stage so that the input current is restricted.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing objects and other objects as well as the characteristic features of the invention are more apparent and more readily understandable from the following description when read in conjunction with the accompanying drawings, in which like reference characters designate the same or similar parts throughout the figures thereof, wherein:

FIG. 1A is a circuit diagram illustrating a conventional Schmitt trigger circuit;

FIG. 1B is a circuit diagram illustrating another conventional Schmitt trigger circuit;

FIG. 2A is a graph illustrating the input-output voltage characteristic and the input voltage-input current characteristic in the circuit of FIG. 1A;

FIG. 2B is a graph illustrating the input-output voltage characteristic and the input voltage-input current characteristic in the circuit of FIG. 1B;

FIG. 3A is a circuit diagram illustrating a Schmitt trigger circuit according to an embodiment of the present invention;

FIG. 3B is a circuit diagram illustrating a Schmitt trigger circuit according to another embodiment of the present invention;

FIGS. 4A and 4B are graphs illustrating the input-output voltage characteristics and the input voltage-input current characteristics in the circuits of FIGS. 3A and 3B, respectively;

FIG. 5A is a plan view of the structure of the input stage of the conventional Schmitt trigger circuit of FIG. 1A or 1B;

FIG. 5B is a cross-sectional view of the structure of the input stage taken along the line V_(b) --V_(b) ' of FIG. 5A;

FIG. 5C is a cross-sectional view taken along the line V_(c) --V_(c) ' of FIG. 5A;

FIG. 6A is a plan view of the structure of the input stage of the Schmitt trigger circuit of FIG. 3A or 3B; and

FIG. 6B is a cross-sectional view taken along the line VI--VI' of FIG. 6A.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Before describing the embodiments of the present invention, two examples of conventional Schmitt trigger circuits are first explained with reference to FIGS. 1A, 1B, 2A and 2B.

FIG. 1A is a circuit diagram illustrating an example of a conventional Schmitt trigger circuit of a true type, in which a low level and a high level of the input voltage correspond to the low level and the high level of the output voltage, respectively. Referring to FIG. 1A, the input stage consists of a pnp transistor T₁ and a load resistor R₁ connected between the emitter of the transistor T₁ and a power supply V_(cc). The output stage consists of npn transistors T₂, T₅ and T₆, load resistors R₃ and R₄ connected between the power supply V_(cc) and the collectors of the transistors T₂ and T₅, respectively, a bias transistor D₂ connected between the emitter of the transistor T₂ and ground, and a bias resistor R₅ connected between the emitter of the transistor T₅ and ground. A Schottky barrier diode D_(s) is connected in the forward direction between the base of the transistor T₁ and the base of the transistor T.sub. 2, for drawing charges on the base of the transistor T₂, or, in other words, for the speeding up of the falling time of the output voltage V_(O). An npn transistor T₃ for providing a hysteresis characteristic and an npn transistor T₄ for controlling the transistor T₃ are disposed between the input stage and the output stage. That is, the collector of the transistor T₃ is connected to a node N₁ between the resistor R₁ and the emitter of the transistor T₁. The emitter of the transistor T₃ is connected to the base of the transistor T₂. The base of the transistor T₃ is connected to the collector of the transistor T₄. The base of the transistor T₄ is connected through a load resistor R₂ to the power supply V_(cc). The emitter of the transistor T₄ is connected to a node N₂ between the resistor R₄ and the collector of the transistor T₅. The transistors T₂ through T₆ are Schottky barrier diodes (SBD) of the clamping type in this example. Further, a transistor D₁, for determining a threshold voltage V_(H), is connected in parallel to the transistor T₃. That is, the base and the collector of the transistor D₁ are short-circuited and are connected to the collector of the transistor T₃ . Also, the emitter of the transistor D₁ is connected to the emitter of the transistor T₃. The transistors D₁ and D₂ are used as pn junction diodes by connecting the bases to the collectors, respectively.

In operation, the base of the transistor T₁ receives an input voltage V_(IN). When the input voltage V_(IN) is low (hereinafter referred to as L), for example, 0 V, the transistor T₁ is in an ON state so that a current I₁ flows through the resistor R₁. During this state, the potential at the anode N₁ is higher than the potential V_(IN) by the amount of only the base-emitter voltage V_(BE) of the transistor T₁. Therefore, the transistor T₂ is in the OFF state. In order to turn on the transistor T₂, the potential at the node N₁ must be higher than the sum of the base-emitter voltages of the transistor D₁, T₂ and D₂, i.e., higher than 3 V_(BE). For this purpose, the input voltage V_(IN) must be higher than 2 V_(BE) because the potential of the base of the transistor T₁ is lower than the potential at the node N₁ by the amount of the base-emitter voltage V_(BE). When the input voltage V_(IN) becomes L to turn off the transistor T₂, the potential at the base of the transistor T₅ becomes high to turn it on. Therefore, a current I₂ flows through the resistor R₂, the transistor T₄, the transistor T₅ and the resistor R₅ so that the potential at the base of the transistor T₆ becomes high to turn it on. As a result, the output voltage V_(o) of this Schmitt trigger circuit becomes L. In this state, a current I₄ does not flow from the base of the transistor T₄ to the base of the transistor T₃. It should be noted that in the actual operation a pull up circuit (not shown) for raising the output voltage V_(o) when the transistor T₆ is in an OFF state is connected between the power supply V_(cc) and the collector of the transistor T₆. The pull up circuit may be a resistor or an active pull up circuit which conducts a current only when the output voltage V_(o) switches.

When the input voltage V_(IN) is raised from L to a value higher than 2V_(BE), the transistor T₂ begins to be turned on at the time when the input voltage V_(IN) reaches 2V_(BE), because a base current I₃ is supplied from the power supply V_(cc) through the resistor R₁, the node N₁, and the transistor D₁ to the base of the transistor T₂. When the transistor T₂ begins to turn on, the base current of the transistor T₅ decreases to turn off the transistor T₅, and accordingly to turn off the transistor T₆. As a result, the current I₂ flowing through the resistor R₂, the transistor T₄ and the transistor T₅ becomes zero, and alternately, a current I₄ begins to flow through the base and collector of the transistor T₄. The current I₄ is supplied to the base of the transistor T₃ to turn it on. During this period the transistor T₃ is on because the collector-emitter voltage V_(CE) of the transistor T₃ is smaller than the base-emitter voltage V_(BE) of the transistor D₁. A base current I₅ flowing through the resistor R₁, the node N₁, and the transistor T₃ is supplied to the base of the transistor T₂. Since the transistors T₅ and T₆ are turned off, the output voltage V_(o) becomes a high voltage (hereinafter referred to as H).

In order to turn off the transistor T₂ when the input voltage V_(IN) changes from H to L, the potential at the node N₁ must be lower than V_(CE) +2V_(BE), which is the sum of the collector-emitter voltage V_(CE) of the transistor T₃ and the two base-emitter voltages 2V_(BE) of the transistors T₂ and D₂. Therefore, the low input threshold voltage V_(L) of the input voltage V_(IN) for changing the output voltage V_(o) from H to L when the input voltage V_(IN) falls from H to L is equal to V_(BE) +V_(CE), while, the high input threshold voltage V_(H), for changing the output voltage V_(o) from L to H when the input voltage V_(IN) rises from L to H, is equal to 2V_(BE), as mentioned before. Accordingly, this Schmitt trigger circuit has input threshold voltages with hysteresis characteristics.

FIG. 2A illustrates the hysteresis characteristic of the input threshold voltages. Referring to the upper portion of FIG. 2A, when the input voltage V_(IN) rises to reach the high input threshold voltage V_(H) equal to 2V_(BE), the output voltage V_(o) turns from L to H. In contrast, when the input voltage V_(IN) falls from H to the low input threshold voltage V_(L) equal to V_(BE) +V_(CE), the output voltage V_(o) turns from H to L . The difference between the high input threshold voltage V_(H) and the low input threshold voltage V_(L) is, as illustrated in FIG. 2A, equal to V_(BE) -V_(CE). Usually, the base-emitter voltage V_(BE) is approximately equal to 0.8 V, and the collector-emitter voltage V_(CE) is approximately equal to 0.2 through 0.3 V. Therefore, the difference between V_(H) and V_(L) is at least 0.4 V. Because of this difference in the input threshold voltages, the Schmitt trigger circuit of FIG. 1A has the advantages of reduced noise sensitivity and of no oscillation near the point of the threshold voltages when compared with a Schmitt trigger circuit having a fixed input threshold voltage, regardless of the rising or falling of the input voltage.

However, in the circuit of FIG. 1A, because of the Schottky barrier diode D_(s) for drawing charges on the base of the transistor T₂, there is a disadvantage that a large current I₆ flows from the power supply V_(cc) through the transistor T₃ and the diode D_(s) to the input when the input voltage falls from H to L, as illustrated in the lower portion of FIG. 2A. More precisely, because the current path through the diode D_(s) to the input is parallel to the current path from the base of the transistor T₂ through the transistor D₂ to the ground, the current I₆ begins to flow when the input voltage V_(IN) falls so that the sum of the input voltage V_(IN) and a voltage drop V_(F) across the diode D_(S) approaches 2V_(BE) which is equal to the sum of the base-emitter voltages of the transistors T₂ and D₂. For example, when the sum of the V_(IN) and V_(F) is equal to 2V_(BE), then, the base current I₅ is equal to the current I₆. As a result, the base current I_(IL) flowing through the base of the transistor T₁ is suddenly but temporarily increased while the input voltage V_(IN) falls from the V_(H) to the V_(L), as illustrated in the lower portion of FIG. 2A. When the input voltage V_(IN) is lowered to a value immediately above the low input threshold voltage V_(L), the input current I_(IL) reaches a maximum value. Then, when the input voltage V_(IN) reaches the V_(L), transistor T₁ is turned on. In this state, the current I₄ does not flow as hereinbefore described, and thus the transistor T₃ is turned off. Therefore, the current I₆ becomes zero. The input current I_(IL) then is reduced to the value I₁ /β, where β is the current amplification factor of the transistor T₁ usually having a value on the order of 20 to 30.

The characteristic feature of a Schmitt trigger circuit employing a pnp transistor at its input stage such as shown in FIG. 1A is its low input current. Therefore, the Schmitt trigger circuit can be driven by using a low current source, such as a CMOS circuit. However, because of the sudden increase in the input current I_(IL) as described above, the current source (not shown), which is to be connected to the input of the Schmitt trigger circuit of FIG. 1A, must be capable of drawing a large current. The CMOS drive circuit can not draw such a large current. Moreover, if a current source is to be connected to a plurality of Schmitt trigger circuits, the problem of not being capable of drawing such a large current becomes more serious.

The problem of conducting a large input current also occurs in a conventional inverted-type Schmitt trigger circuit, an example of which is illustrated in FIG. 1B. The difference between the circuits of FIGS. 1A and 1B is that instead of connecting a circuit comprised of the resistors R₂ and R₃, and the transistors T₂, T₄ and D₂ in the circuit of FIG. 1A, a circuit comprising resistors R₆ and R₇, and npn transistors T₇, T₈ and D₃ is connected between the input stage and the output stage in FIG. 1B. The resistor R₆ is connected between the power supply V_(cc) and the collector of the transistor T₇. The base of the transistor T₃ is also connected to the collector of the transistor T₇. The resistor R₇ is connected between the power supply V_(cc) and the base of the transistor T₈. The collector of the transistor T₈ is connected to the base of the transistor T₇. The emitter of the transistor T₈ is connected to the collector of the transistor T₅. The emitter of the transistor T₇ is connected to the collector and the base of the transistor D₃. The emitter of the transistor D₃ is connected to the ground.

The upper portion of FIG. 2B illustrates the hysteresis characteristic of the input threshold voltages of the circuit of FIG. 1B. In operation of the circuit of FIG. 1B, when the input voltage falls from H to L so that the input voltage V_(IN) becomes lower than the low input threshold voltage V_(L), the transistor T₁ turns on. When the transistor T₁ is in the ON state, the transistor T₅ is not supplied with its base current because the potential at the node N₁ is too low. Therefore, the transistor T₅ and accordingly the transistor T₆ are in their OFF states so that the output voltage V_(o) is H. In contrast, when the input voltage V_(IN) rises from L to H, the transistor T₁ turns off when the input voltage V_(IN) becomes higher than the high input threshold voltage V_(H). When the transistor T₁ is in the OFF state, the transistors T₅ and T₆ are in their ON states so that the output voltage V_(o) is L. The transistor T₃ is in an OFF state when the input voltage V_(IN) is L, because the transistor T₅ is in its OFF state, so that the potential at the node N₂ is equal to V_(cc), and accordingly, a current flows through the resistor R₇ and the base-collector of the transistor T₈ to provide a base current to the transistor T₇ to turn it on. Therefore, the transistor T₃ does not receive a base current when the input voltage V_(IN) is L. In contrast, when the input voltage V_(IN) is H, the transistor T₃ is in an ON state because the transistor T₅ is in an ON state so that the potential at the node N₂ is decreased. Therefore, a current flows through the base-emitter of the transistor T₈, the transistor T₅ and the resistor R₅. Accordingly, the transistor T.sub. 7 does not receive the base current. As a result, the transistor T₃ receives its base current from the power supply V_(cc) through the resistor R₆, when the input voltage V_(IN) is H. As in the circuit of FIG. 1A, the current I₆ flowing through the transistor T₃ and the diode D_(s) is also generated in the circuit of FIG. 1B when the input voltage V_(IN) falls from H to L, as illustrated in the lower portion of FIG. 2B.

According to the present invention, instead of the Schottky barrier diode D_(s) used in the conventional circuit of FIG. 1A of FIG. 1B, a multi-emitter transistor is employed for suppressing such a large input current during the falling of the input voltage. Embodiments of the present invention are described below.

FIG. 3A is a circuit diagram illustrating a Schmitt trigger circuit according to one embodiment of the present inventon. The circuit of FIG. 3A is of a true type in which the L level and the H level of the input voltage V_(IN) correspond to the L level and the H level of the output voltage V_(o), respectively. The difference between the conventional circuit shown in FIG. 1A and the circuit shown in FIG. 3A is that, the Schottky barrier diode D_(s) used in the circuit of FIG. 1A is not employed in the circuit of FIG. 3A, and instead of the pnp transistor T₁, a multi-emitter pnp transistor T₁₁ is employed in FIG. 3A. The first emitter E₁ of the transistor T₁₁ is connected to the load resistor R₁. The second emitter E₂ of the transistor T₁₁ is connected to the base of the transistor T₂ as well as to the emitters of the transistors D₁ and T₃. By this construction, the multi-emitter transistor T₁₁ not only operates in a way similar to the transistor T₁ in FIG. 1A, but also operates to solve the problem caused by the circuit of FIG. 1A.

The operation of the circuit of FIG. 3A is described with reference to FIG. 4A, which is a graph illustrating the input-output voltage characteristic and the input voltage-input current characteristic. Referring to the upper portion of FIG. 4A, the hysteresis characteristic of the input threshold voltage is the same as that illustrated in FIG. 2A for the conventional circuit of FIG. 1A because a first transistor T_(a) of the multi-emitter transistor T₁₁ consisting of the first emitter E₁, the base and the collector, operates in the same way as the transistor T₁ in the circuit of FIG. 1A. However, as can be seen from the lower portion of FIG. 4A, a sudden increase in the input current I_(IN) is not generated while the input voltge V_(IN) falls from H to L. That is, during the period in which the input voltage is falling from V_(H) to the V_(L), transistor T_(a) begins to turn on. Therefore, the potential at the node N₁ is higher than the input voltage V_(IN) by the amount of the voltage V_(BE) between the base and the first emitter E₁ of the transistor T_(a). In contrast, if a current such as the current I₆ in FIG. 1A could flow through the transistor T₃ and the second emitter E₂ to the base of a second transistor T_(b) which consists of the second emitter E₂, the base and the collector of the transistor T₁₁, then the voltage drop between the collector of the transistor T₃ and the base of the transistor T₁₁ would be equal to the sum of V_(CE) and V_(BE) ', where V_(BE) ' is the base-second emitter E₂ voltage of the transistor T_(b). Since V_(BE) ' is nearly equal to V_(BE), V_(BE) '+V_(CE) is greater than V_(BE). Therefore, the current path through the transistor T₃ and the second emitter E₂ to the base can not substantially exist during this period. As a result, the input current I_(IN) is equal to I₁ /β_(a), where β_(a) is the current amplification factor of the transistor T_(a). If a current flows through the transistor T_(b) in a transient state of the transistor T₁₁, the current will be decreased by a factor of β_(b), where β_(b) is the current amplification factor of the transistor T_(b), and nearly equal to β_(a). In either case, I_(IL) is substantially equal to I₁ /β_(a).

Further, the transistor T_(b) in the multi-emitter transistor T₁₁ acts to draw charges to ground from the base of the transistor T₂ for the speeding up of the operation when the input voltage falls as the Schottky barrier diode D_(s) did in the circuit of FIG. 1A. That is, when the input voltage V_(IN) falls to a value lower than the low input threshold voltage V_(L), the transistor T_(b) is completely turned on so that charges on the base of the transistor T₂ are drawn to the second emitter E₂ as a current I₇. Due to the current amplification factor β_(b) of the transistor T_(b), the current I₇ is reduced to I₇ /β_(b) at the base of the transistor T_(b).

FIG. 3B is a circuit diagram illustrating a Schmitt trigger circuit according to another embodiment of the present invention. The circuit of FIG. 3B is of an inverted type in which the L level and the H level of the input voltage V_(IN) correspond to the H level and the L level of the output voltage V_(o), respectively. Referring to FIG. 3B, the multi-emitter transistor T₁₁ is also employed. The other portion is the same as the corresponding portion in the conventional circuit of FIG. 1B. The operation of the circuit of FIG. 3B may be easily understood by those skilled in the art from the foregoing description and by referring to FIG. 4B, which is a graph illustrating the input-output voltage characteristic and the input voltage-input current characteristic in the circuit of FIG. 3B. Therefore, further description with respect to the circuit of FIG. 4B is not provided here.

The multi-emitter transistor T₁₁ can easily be integrated with the other elements on a semiconductor substrate without serious design alteration of the conventional Schmitt trigger circuit of FIG. 1A or 1B. This is easily understood from the following description with reference to FIGS. 5A, 5B, 5C, 6A and 6B. FIG. 5A is a plan view of the structure of the transistor T₁ and the Schottky barrier diode D_(s) illustrated in the conventional Schmitt trigger circuit of FIG. 1A or 1B. FIGS. 5B and 5C are cross-sectional views taken along the lines V_(b) --V_(b) ' and V_(c) --V_(c) ' of FIG. 5A. Referring to FIG. 5A, the electrodes 9, 7, and 13 of the collector C, the emitter E and the base B of the transistor T₁, respectively, and the anode electrode 8 of the Schottky barrier diode D_(s) are disposed on a surface of an insulating layer 3. Referring to FIG. 5B, in an n⁻ -type epitaxial layer 1, αp⁺ -type isolation region 2 is formed for electrically separating the transistor T₁ and the diode D_(s) from the other elements on the Schmitt trigger circuit. On the n⁻ -type epitaxial layer 1 and the isolation region 2, the insulation layer 3 of, for example, SiO₂, is formed. A window 4 for forming the emitter E of the transistor T₁ and a window 5 for forming the Schottky barrier of the diode D_(s) is provided in the insulation layer 3. A p⁺ -type diffusion region 6 is formed by diffusing p⁺ -type impurity atoms through the window 4. On the window 4, an aluminum electrode 7 for the emitter E of the transistor T₁ is formed. On the window 5, an aluminum electrode 8 for the anode of the diode D_(s) is formed. The Schottky barrier of the diode D_(s) is formed by the contact of the aluminum layer 8 to the n⁻ -type epitaxial layer 1. Referring to FIG. 5C, an aluminum electrode 9 for the collector C is formed on a window 10 which makes an electrical contact joining the p⁺ -type region 2 to the alumimum electrode 9. An n⁺ -type diffusion region 11 for the base contact is formed by diffusing n⁺ -type impurity atoms through a window 12. On the window 12, an aluminum electrode 13 is formed to be the base electrode.

FIG. 6A is a plan view of the structure of the multi-emitter transistor T₁₁ illustrated in FIG. 3A or 3B. FIG. 6B is a cross-sectional view taken along the line VI--VI' of FIG. 6A. The only difference between the structures of the conventional circuit illustrated in FIGS. 5A through 5C and of the embodiment of the present invention illustrated in FIGS. 6A and 6B is that in FIG. 6B a p⁺ -type diffusion region 14 is formed by diffusing p⁺ -type impurity atoms through the window 5. The region 14 acts as the second emitter E₂. While the region 6 acts as the first emitter E₁. Because no additional process step is required for forming the p⁺ -type region 14, the multi-emitter transistor T₁₁ can easily be integrated with the other elements of the Schmitt trigger circuit.

Instead of forming the multi-emitter transistor T₁₁, a capacitor or a pn junction diode may be formed in place of the Schottky barrier diode D_(s). However, if a capacitor or a pn junction diode is formed for suppressing the sudden increase in the input current, additional steps of, for example, forming isolation regions, are required, so that the manufacturing steps become complicated and the chip area is increased. Therefore, in view of the circuit integration, the multi-emitter transistor has the distinct advantage of being able to suppress any sudden increase in the input current compared to a capacitor or a pn junction diode.

From the foregoing description, it will be apparent that according to the present invention in a Schmitt trigger circuit having input threshold voltages with a hysteresis characteristic for reducing input noise sensitivity and for preventing oscillation at a point near the threshold voltages, a sudden increase in the input current during falling of the input voltage is greatly suppressed by employing a multi-emitter transistor in the input stage.

It is apparent that the present invention is not restricted to the illustrated embodiments but various changes and modifications are possible without departing from the basic scope of the invention. For example, although the transistors T₂ through T₈ are of Schottky barrier clamping type in the foregoing embodiments, these transistors may be of any bipolar type. Also, although no buried layer is illustrated in the structure of the conventional Schmitt trigger circuit, a buried layer is usually formed for reducing a DC resistance of the diode D_(s). The structure of the Schmitt trigger circuit according to the present invention may of course include such a buried layer. 

I claim:
 1. A Schmitt trigger circuit having an input-voltage hysteresis characteristic, comprising:an input stage comprising a first transistor which is a multi-emitter transistor having first and second emitters, and a first load resistor connected to said first emitter at a node; an output stage comprising a second transistor having a base connected to said second emitter of said first transistor; a first pn junction diode having an anode connected to said node and a cathode connected to the base of said second transistor, for turning on said second transistor by applying a voltage lower than the voltage at said node by the amount of the voltage drop across said first pn junction diode; a third transistor having a collector and an emitter connected to said first pn junction diode, for providing said input-voltage hysteresis characteristic; and a switching circuit, connected to said second and third transistors, for turning on said third transistor in response to a turning on of said second transistor.
 2. A Schmitt trigger circuit as set forth in claim 1, wherein said first transistor is a multi-emitter pnp type transistor further comprising a base connected for receiving the input voltage of the Schmitt trigger circuit, and a collector connected to ground.
 3. A Schmitt trigger circuit as set forth in claim 2, further comprising a power supply, wherein said first load resistor is connected between said power supply and said node.
 4. A Schmitt trigger circuit as set forth in claim 3, wherein said first pn junction diode comprises an npn transistor having an emitter functioning as said cathode and having a base and a collector which are short-circuited and connected to said node to function as said anode, wherein said third transistor has a collector connected to the collector of said first pn junction diode at said node, and wherein said third transistor has an emitter connected to the emitter of said first pn junction diode and to said second emitter of said first transistor.
 5. A Schmitt trigger circuit as set forth in claim 4, wherein said second and third transistors are npn type transistors.
 6. A Schmitt trigger circuit as set forth in claim 5, wherein said second and third transistors are Schottky barrier clamping type transistors.
 7. A Schmitt trigger circuit as set forth in claim 5 or 6, wherein said switching circuit comprises:a second load resistor connected to said power supply; and a fourth transistor which is an npn transistor having a base connected to said load resistor, and having a collector connected to the base of said third transistor.
 8. A Schmitt trigger circuit as set forth in claim 7, wherein said Schmitt trigger circuit further comprises:a second pn junction diode connected between the emitter of said second transistor and ground; and a third load resistor connected to said power supply and the collector of said second transistor.
 9. A Schmitt trigger circuit as set forth in claim 7, wherein said output stage further comprises:a third load resistor having a first terminal connected to said power supply and having a second terminal; a fifth transistor which is npn transistor having a base connected to the collector of said second transistor, having a collector connected to the emitter of said fourth transistor and to said second terminal of said third load resistor, and having an emitter; and a bias resistor connected to the emitter of said fifth transistor and to ground.
 10. A Schmitt trigger circuit as set forth in claim 9, wherein said output stage further comprises a sixth transistor which is npn transistor having a base connected to the emitter of said fifth transistor, having a collector connected as an output terminal of said Schmitt trigger circuit, and having an emitter connected to ground.
 11. A Schmitt trigger circuit as set forth in claim 5 or 6, wherein said switching circuit comprises:a second load resistor having a first terminal connected to said power supply and having a second terminal; a third load resistor having a first terminal connected to said power supply and having a second terminal; a fourth transistor which is an npn transistor having a collector connected to the base of said third transistor and to the second terminal of said second load resistor, having a base, and having an emitter; said Schmitt trigger circuit further comprising:a second pn junction diode connected to the emitter of said fourth transistor and to ground; a fifth transistor which is an npn transistor having a base connected to said second terminal of said third load resistor, having an emitter connected to the collector of said second transistor, and having a collector connected to the base of said fourth transistor.
 12. A Schmitt trigger circuit as set forth in claim 11, further comprising:a bias resistor connected to ground; and a fourth load resistor having a first terminal connected to said power supply and having a second terminal, wherein said second transistor has a collector connected to the second terminal of said fourth load resistor, and an emitter connected to said bias resistor.
 13. A Schmitt trigger circuit as set forth in claim 12, wherein said output stage further comprises a sixth transistor which is an npn transistor having a base connected to the emitter of said second transistor, having a collector connected as an output terminal of said Schmitt trigger circuit, and having an emitter connected to ground. 